Kuhn’s heroes: Five paradigm-busting revolutions

Thomas Kuhn’s The Structure of Scientific Revolutions wasn’t an instant hit when it was published in 1962, but it went on to notch up sales of more than a million, up there with Richard Dawkins’s The Selfish Gene. It’s now seen as one of the most important books of the 20th century. Get a flavour of it as we look at the four big revolutions that Kuhn wrote about – and one that he didn’t. Julian Richards

The Copernican revolution

Medieval Europe's astronomy was defined by Greco-Roman scholar Ptolemy, who thought the sun and planets orbited Earth. But he could see that simple orbits could not account for the movements of Mars, Jupiter and Saturn, and proposed they moved in "epicycles", orbiting a point that was itself orbiting Earth. Later astronomers devised ever more complicated systems to account for the anomalies in their observations – a typical forerunner of a Kuhnian scientific revolution.

In the early 1500s, Nicolaus Copernicus staked out a new paradigm: Earth and the planets moved around the sun, not vice versa. But his theory was far from complete – another feature invoked by Kuhn for his revolutions. Copernicus assumed orbits were perfectly circular so he had to invoke epicycles.

Not everyone accepted Copernicus: astronomer Tycho Brahe proposed a hybrid of Earth and sun-centred models. This slow spread of a new paradigm was typical of Kuhn's scientific revolutions.

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The Newtonian revolution

Isaac Newton sought laws that would apply at all times and in all places. He created a "revolution" out of the contradictory views on the nature of light: Kuhn called it "the first nearly uniformly accepted paradigm for physical optics".

In Principia Mathematica (1687), Newton suggested the force of gravity "would be the same for all types of matter at all positions in the universe". But it would be a century before anyone made equipment capable of measuring this constant.

Meanwhile, many astronomers found the moon's movements refused to conform to Newton's laws. As Kuhn describes it: "some… suggested replacing the inverse square law with a law that deviated from it at small distances. To do that, however, would have been to… define a new puzzle… and not to solve the old one." In the event, the scientists hung onto Newton until they managed to make the moon "fit".

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The Lavoisier revolution

Until Antoine Lavoisier, chemists thought combustible materials burned by releasing a substance called phlogiston during combustion. Lavoisier was first to realise that things burn by incorporating a gas that is part of air – something he named "oxygen" in 1778.

Does that mean Lavoisier discovered oxygen? The answer, said Kuhn, is more complicated than the question suggests. "Discovering a new sort of phenomenon is necessarily a complex event," he wrote, "one which involves recognizing both that something is and what it is."

Even Lavoisier didn't get it right: he thought that oxygen was an atomic "principle of acidity" that formed a gas only when combined with "caloric", a hypothetical fluid thought to be responsible for heat. Still, his insight into combustion trumped the phlogiston theory and opened the way to a new world of chemical research.

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The Einsteinian revolution

Kuhn saw "normal" science as an exercise in "puzzle-solving". When scientists encounter phenomena that contradict the dominant paradigm inherent in "normal" science, they try hard to make these anomalies fit the norm. Before Albert Einstein's special theory of relativity (1905), the prevailing view was that space was filled with an invisible substance, "ether", through which light travelled.

Unfortunately, no one had ever managed to detect it – and "normal" science had been forced to provide many ingenious explanations of this failure.

Einstein did more than provide an alternative to the ether theory, however. "Within the new paradigm, old terms, concepts, and experiments fall into new relationships," wrote Kuhn. "To make the transition to Einstein's universe, the whole conceptual web whose strands are space, time, matter, force, and so on, had to be shifted and laid down again on nature whole."

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The Darwinian revolution

Kuhn, a physicist, devoted few words to Darwin's transformation of life sciences. But he argued that all scientists should learn one of the lessons of the Darwinian revolution – that progress doesn't have to have a goal.

"Though evolution… did encounter resistance, particularly from some religious groups, it was by no means the greatest of the difficulties the Darwinians faced," wrote Kuhn. "All the well-known pre-Darwinian evolutionary theories… had taken evolution to be a goal-directed process. The 'idea' of man and… contemporary flora and fauna was thought to have been present from the first creation of life, perhaps in the mind of God….

"For many… the abolition of that teleological kind of evolution was the most significant and least palatable of Darwin's suggestions. The Origin of Species recognized no goal set either by God or nature…. What could 'evolution,' 'development,' and 'progress' mean in the absence of a specified goal?"